Police are warning bank customers to be on their guard following a number of incidents involving thieves using a device to crash cash machines.

The plastic loop is inserted into bank machines when a cash card is inserted, before the thieves move in for the "sting".

An estimated £21m is stolen by using devices such as the loop - a fraction of the £400m of till machine card theft reported each year.

Credit and debit card purchases are protected by encryption

South Wales Police joined other forces in warning the technique of the plastic loop is on the increase by organised gangs working in towns and cities across the UK.

Once a card is trapped in the plastic loop, thieves acting as casual passers by, offer their help and advise the customer to tap in their Personal Identification Number (Pin) number while pressing the cancel button.

But when the advice fails to produce the card and the customer walks off thinking the machine has swallowed the card, the thief then removes it.

Because the customer has casually keyed in their pin number, the thief then has access to the bank account.

Mr Sweet had £110 taken out of his account

In Merthyr Tydfil last week, South Wales Police reported four incidents and there have been 17 across the force area.

There have also been cases reported in London and other towns and cities.

John Sweet, 25, from Pontypridd, south Wales, was a victim of the sting last month in Merthyr Tydfil, when £110 was taken from his account, after two "unusually helpful" young women stopped to assist him.

Refund

He reported the incident to his bank, but he was told he could lose his card for going overdrawn.

The issue of liability for the fraud, if proven by the bank, lies with the bank, confirmed a spokeswoman for Barclays Bank.

"If a customer reports an incident, we will investigate it. If fraud is proven, we will refund the money," she said.

"We would recommend people remain vigilant."

Gemma Smith, from banking industry body Apacs, said the issue of fraud was being dealt with on several levels, including improving till machine security levels.

"This card-trapping device is on the increase and we are concerned by it," she said.

PC Richie Gardiner, of Merthyr Tydfil crime prevention unit, said the advice to customers was never give out your Pin number under any circumstances.